Metamorphic Rocks

Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form ".The protolith is subjected to heat and pressure (temperatures greater than 150 to 200 °C) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above. 

Some examples of metamorphic rocks are gneiss, slate, marble and schist.

Formation

They can be formed by tectonic processes such as continental collisions which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior.Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.

Metamorphic minerals

Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, kyanite, staurolite, andalusite, and some garnet.

Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they were formed.

Classification

The two basic types of metamorphic rocks are Foliated metamorphic rock and Non-Foliated metamorphic rock.

Foliated metamorphic rock:

A foliated metamorphic rock will have banded minerals. The mineral flakes will appear to be parallel to the rock and will look layered. When a foliated rock breaks, a thin rock fragment will result.

Foliated rocks are most often formed from mudstones and contain "fine-grained" or "platy" minerals that are usually too small to see with the naked eye; although some can be seen without aid. Examples of foliated rocks are slate, phyllite and schist.

Slate

Non-foliated metamorphic rock:

A nonfoliated rock will have almost the opposite texture. The minerals will appear to be randomly oriented without obvious banding and have a granular appearance. Unlike a foliated rock, there will be no layers and they will not flake apart into thin layers when broken.
Nonfoliated rocks contain more coarse grained minerals and generally have a random shape. Because of this, these rocks are very granular in appearance. Examples of nonfoliated rocks are quartzite, marble and anthracite coal.

Marble

Uses:

1. Marble is mostly used for furnishing surfaces due to its lustrous color and glossy appearance.
2. Slate is commonly used for roofing and flooring or it can also be crushed and used in paints.
3. Quartzite is a decorative stone and may be used to cover walls, as roofing tiles, as flooring, and stair steps. Crushed quartzite is sometimes used in road construction
4. Schist rock is used for decorative rock walls, pillars, paving, jewelry and other pieces.
5. Gneiss has many uses as a building material such as flooring, ornamental stones, gravestones, facing stones on buildings and work surfaces.